This invention relates to a broadly modulated radiant gas burner that yields minimal emissions of air-pollutants, especially nitrogen oxides (NOx). More particularly, the burner face of this invention is a porous mat of metal and/or ceramic fibers which is divided into segments that can be individually fired.
Radiant, surface-combustion gas burners are fed fuel gas admixed with enough air to ensure complete combustion of the fuel gas. Because these burners function without secondary air, their modulation of heat output is limited. Yet, there are important uses of surface-combustion gas burners in tight spaces, such as in the casings of gas turbines, where adding spare burners to increase heat delivery is not a practical solution to broad heating modulation.
Assignee""s U.S. Pat. No. 6,199,364 to Kendall et al discloses compact surface-stabilized gas burners that are well suited for use with gas turbines. Surface-stabilized gas burners are therein defined as having burner faces with dual porosities so that surface combustion from the lower porosity areas serves to keep the blue flames from the higher porosity areas attached to the burner face when fired at rates of at least about 500,000 BTU/hr/sf (British Thermal Units per hour per square foot) of burner face.
A principal object of this invention is to provide compact surface-stabilized gas burners featuring a broad range of heat delivery.
Another important object is to provide such surface-stabilized gas burners with internal walls that divide each burner into two or more segments that can be individually and independently fired to vary the thermal output, while maintaining the adiabatic flame temperature of the fired segments in a range yielding low emissions.
Still another object is to provide segmented surface-stabilized gas burners that are simple in construction as well as operation.
These and other features and advantages of the invention will be apparent from the description which follows.
Basically, the segmented surface-stabilized gas burner of this invention which has a combustion surface formed of metal and/or ceramic fibers may have a unitary body with internal partitions to provide independent burner segments, or it may have two or more burner modules that are compactly fitted together.
U.S. Pat. No. 4,543,940 to Krill et al describes a segmented radiant surface burner formed of large cylindrical segments that are bolted together in axial alignment. This arrangement of large burner segments was conceived to fit the peculiar shape of combustion chambers of fire tube boilers. The serial alignment involves sealing between the abutted ends of contiguous burner sections and requires an individual duct to supply fuel gas and air to each burner segment. The complex ducting of fuel gas and air to each burner segment is antithetical to this invention""s objective of burner compactness that is essential to burners used with gas turbines.
The combustion surface may be formed of ceramic fibers as taught by U.S. Pat. No. 4,746,287 to Lannutti, of metal fibers as set forth in U.S. Pat. No. 4,597,734 to McCausland, or of mixed metal and ceramic fibers according to U.S. Pat. No. 5,326,631 to Carswell et al. For high surface firing rates, say, at least about 500,000 BTU/hr/sf of burner face, a rigid but porous mat of sintered metal fibers with interspersed bands or areas of perforations is preferred. Such a burner face is shown in FIG. 1 of U.S. Pat. No. 5,439,372 to Duret et al. Still another form of porous metal fiber mat sold by N.V. Acotech S.A. of Zwevegem, Belgium, is a knitted fabric made with a yarn formed of metal fibers. In the rigid porous and perforated burner of Duret et al, radiant surface combustion is interspersed with blue flame combustion from the perforations. Similarly, the yarn of the knitted metal fiber fabric provides radiant surface combustion and the interstices of the knitted fabric naturally provide interspersed spots of increased porosity that yield blue flames.
At the aforesaid high surface firing rates, the flames from the areas of increased porosity produce such intense non-surface radiation that the normal surface radiation from the areas of lower porosity disappears. However, the dual porosities make it possible to maintain surface-stabilized combustion, i.e., surface combustion stabilizing blue flames attached to the burner face. Burner faces with dual porosities are referred to as surface-stabilized burners. With such burners, flaming is so compact that visually a zone of strong infrared radiation appears suspended close to the burner face. It is noteworthy that with at least about 40% excess air, surface-stabilized combustion yields combustion products containing low emissions as little as 2 ppm (parts per million) NOx and not more than 10 ppm CO and UHC (unburned hydrocarbons), combined.
In as much as the segmented burner of this invention is particularly valuable in uses where the combustion zone is spatially limited, it is seldom a flat burner. Cylindrical burner faces and variations thereof, e.g., tapered or conical, are the usual forms of the segmented burner.
The burner segments which fit together may be designed to deliver equal quantities of heat, but it is usually advantageous to have segments of unequal heat delivery capacities. For example, a two-segment burner, can have one segment with 60% and the other segment with 40% of the total heat delivery capacity of the burner. Such unequal segments permit greater heat delivery modulation than if the burner had two equal segments. The same is true of three-segment burners. Three segments of 55%, 35% and 10% of heat delivery capacity permit greater modulation of heat delivery than is possible with three segments of equal heat delivery capacity.